1. Field of the Invention
The invention is directed generally to nanotube sensors and in particular to nanotube sensors with selective passivation of nanotube-conductor contacts and the nanotubes themselves and methods of forming same.
2. Description of the Related Art
Chemical and biological sensors that use nanotube circuits have been reported in the literature. In general, these sensors include a nanotube or nanotubes in contact with electrodes, thus forming a circuit for current flow. It is generally believed that sensing occurs when analytes interact with the exposed nanotube.
Dai et al. (PCT Publication No. WO 01/44796 A1) described a nanotube sensing device which had nanotubes grown from catalyst islands and metal electrodes that covered fully the catalyst islands. The ends of the nanotubes were embedded in the catalyst islands within the metal electrodes.
Lieber et al. (PCT Publication No WO 02/48701 A2) described a nanowire sensing device that was particularly adapted for sensing analytes in fluids delivered through a microchannel to the nanowire which was connected to two metal electrodes.
Nanotube sensors have been reported to respond to chemical species such as ammonia (Kong, J., et al., Science, 287, 622 (2000)). These sensors exhibited a fast response and a substantially slower recovery. Other researchers have found that nanotube sensors are unpredictable in their response to ammonia. For some sensors the resistance went up in response to the analyte, for some it went down, and the magnitude of the response was variable as well. The sensors had both nanotubes and electrode/nanotube contacts that were exposed to the surrounding atmosphere.
In semiconductor device technology, it is well known that metal contacts to silicon can be sensitive and problematic. Schottky barriers at metal/semiconductor junctions create a large number of surface states that are very sensitive to the surrounding environment. For this reason, among others, metal/semiconductor contacts have been passivated by covering the device with a layer of insulating material. Thus the contacts do not come into contact with the surrounding environment, nor is any other part of the device exposed to the surrounding environment.
Scientists at IBM have conducted research on nanotube transistor devices that were configured to act as electronic devices, not as sensing devices. Nanotube device characteristics changed depending on their exposure to oxygen. As-made devices generally exhibited p-type transistor characteristics. Derycke, et al. (Appl. Phys. Lett. 80, 2773 (2002)) reported that their devices changed to n-type transistors after heating in vacuum. The change could be reversed only be exposing the devices to oxygen. They attributed this behavior to removal of adsorbed oxygen from the contacts. Their key finding was that the main effect of oxygen adsorption was to modify the barriers at the metal-semiconductor contacts.
Avouris has reported (Accounts of Chemical Research, ASAP Article 10.1021/ar010152e S0001-4842(01)00152-2 Web Release Date: Jul. 31, 2002.) enclosing p-type carbon nanotube field effect transistors in SiO2 and annealing them at 700° C. in an inert gas or in vacuum. Subsequent electrical measurements showed that the devices had become ambipolar (with conduction by either holes or electrons) transistors. The ambipolar behavior was observed only when the device was passivated by a film of SiO2 before the thermal annealing. At the high annealing temperature, O2 can diffuse through the oxide and desorb. Upon cooling, however, the SiO2 film protects the device from oxygen. Avouris concluded that the observed electrical behavior was dominated by oxygen's effect on the Schottky barriers at the metal-nanotube junctions.
Nanostructure sensing devices are most often used to detect a species of interest in a surrounding environment. Electrical signals from the nanostructure sensing devices can be measured before and after exposure to the environment. Changes in measured signals can be correlated to detection of a species. Total passivation of a nanostructure sensing device could result in gross underreporting of detection events if the species of interest cannot diffuse through the passivation layer and reach the nanostructure sensing device. On the other hand, very sensitive Schottky barriers may respond to species that are not of interest, such as moisture or oxygen. Large electrical responses from Schottky barriers could overwhelm smaller nanostructure responses from detection of species of interest.
It would be useful to understand better analyte interactions at Schottky barriers and at nanostructures. This understanding could be used to design nanostructure sensing devices in various configurations to exploit the special sensing characteristics of both Schottky barriers and nanostructures in nanostructure sensing devices.